Transmitter for transmitting signals over radio channels and method for transmitting signals over radio channels

ABSTRACT

A transmitter for sending signals over wireless channels and a method of sending signals over wireless channels are described. The transmitter and the method are used to determine a transfer characteristic of an amplifier ( 8 ) in the transmitter. Test signals are input at preset times into OFDM signals to be transmitted, and then test signals amplified by the amplifier ( 8 ) are compared with test signals buffered in a measurement module ( 12 ) to determine the transfer characteristic of the amplifier ( 8 ). This transfer characteristic of the amplifier ( 8 ) is used by a predistorter ( 4 ) to predistort the signals according to this transfer characteristic. The test signal generated by a signal generator ( 13 ) is input into a synchronization symbol, with the amplitude of the test signal being increased incrementally or being large enough to determine the transfer properties of the amplifier ( 8 ). The test signal has an envelope which is not dependent on time.

BACKGROUND INFORMATION

The present invention is based on a transmitter for sending signals overwireless channels and a method of sending signals over wireless channelsaccording to the definition of the species of the independent patentclaims.

Predistortion of OFDM signals (OFDM=orthogonal frequency divisionmultiplex), specifically according to the transfer properties of theamplifier in the transmitter, is already known from M. Schrader and N.Hentati “Reduktion von Auβerbandstrahlung von Sendestufen imDAB-COFDM-System [Reducing out-of-band radiation of transmitter stagesin the DAB-COFDM system]” OFDM Fachgespräche, September 1998,Braunschweig, printed in the conference volume. This is necessarybecause OFDM signals require good linearity of the amplifier in thetransmitter due to the great difference between the large and smallamplitudes occurring in OFDM signals, i.e., the dynamics or amplitudevariance, because all OFDM signal amplitudes are to be amplifiedlinearly. The article referenced above describes a feedback system forpredistortion, where a portion of the amplified OFDM signal is fed backand compared with a buffered OFDM signal to determine the transferproperties of the amplifier in the transmitter. The buffered OFDM signalis the OFDM signal which is then amplified and fed back. Since theproperties of the OFDM signal are very similar to those of a noisesignal, this requires high-quality synchronization of the buffered OFDMsignal and the amplified OFDM signal.

ADVANTAGES OF THE INVENTION

The transmitter according to the present invention for sending signalsover wireless channels and the method according to the present inventionof sending signals over wireless channels having the features of theindependent patent claims has the advantage that a test signal is inputinto the OFDM signal to determine the transfer properties of theamplifier. This has the advantage that the complete transfer propertiesof the amplifier are determined, and thus better predistortion of theOFDM signals is possible.

Another advantage is that through the use of a suitable test signal,synchronization can be performed more easily with the feedback testsignal and a buffered test signal.

Another advantage is that the test signal is only input at presetintervals, thus minimizing any negative effect on the signal sent due tothis input.

Advantageous refinements of and improvements on the transmitter and themethod characterized in the independent patent claims are possiblethrough the measures characterized in the dependent claims.

It is especially advantageous that differential phase modulation,preferably differential quadrature phase shift keying, is used as themodulation method for imposing information on the OFDM signals. This hasthe advantage that the receiver need not determine an absolute phase butinstead need only determine the phase shift between the signals fordemodulation.

In addition, another advantage is that the test signal has an envelopewhich is not dependent on time. This minimizes the influence of the testsignal on the measurement itself.

It is advantageous that the amplitude of the test signal is increasedincrementally to determine the transfer properties of the amplifier. Atransfer characteristic of the amplifier is thus determinedincrementally.

Another advantage is that the test signal has an amplitude such that thecontrol settings of the amplifier are fully adjusted with it. This savestime and bandwidth in determining the transfer characteristic. To thendetermine individual sections of the transfer characteristic, samples ofthis test signal are used to determine the transfer properties.

In addition, it is advantageous that the test signal is input into asynchronization symbol of the signals, so that no bandwidth is lost foruseful data.

DRAWING

Embodiments of the present invention are illustrated in the drawing andare explained in greater detail in the following description.

FIG. 1 shows a block diagram of an OFDM transmitter according to thepresent invention;

FIG. 2 shows a DAB frame, and

FIG. 3 shows a method according to the present invention for sendingsignals over wireless channels.

DESCRIPTION OF EMBODIMENTS

Orthogonal frequency division multiplex (OFDM) is a well-known andsuccessful method for mobile wireless applications. In OFDM, signals tobe transmitted are distributed among multiple subcarriers, eachsubcarrier having a certain frequency interval from the others, so thatthe signals distributed among the subcarriers do not cause mutualinterference. This is described as orthogonal.

OFDM is therefore used for digital wireless transmission methods, inparticular for mobile reception, e.g., by way of car radios, includingDAB (Digital Audio Broadcasting), DVB (Digital Video Broadcasting) andDRM (Digital Radio Mondial). These wireless transmission methods profitfrom the property of OFDM that, when frequency-selective damping occurs,only a small portion of the wireless signal transmitted hasinterference, because the wireless signal has been distributed among aplurality of frequencies and the only signal component containinginterference is transmitted at a frequency at which there is strongdamping. The signal component containing interference is corrected byerror detecting and correcting measures. Such error detecting andcorrecting measures include error detecting and correcting codes such asblock codes or convolution codes.

In OFDM, after the signals to be transmitted have been distributed amongthe subcarriers, they are added up in the time interval of thedistributed signals, and the amplitudes may be added up in such a waythat the amplitudes of the superimposed signal assume a very large valueat certain times and also assume a very small value at other times. Thisdepends on the phase relationship of the signal components being added,namely whether the signals are added constructively or destructively. Anamplifier in the transmitter has the function of amplifying allamplitudes equally, so there is no nonlinear distortion.

Predistortion is used to take into account the transfer properties ofthe amplifier in the transmitter. To determine a transfer characteristicof the amplifier, a signal amplified by the amplifier must be comparedwith the original signal. The OFDM signal is a challenge due to theuncorrelated sequence of amplitudes occurring due to addition of theindividual signal components, because it is difficult to synchronize theoriginal OFDM signal with the amplified OFDM signal.

For amplification of OFDM signals, the amplifier should be operated onlyin the linear range. If a signal transmitted at a certain frequency isapplied to a nonlinear characteristic curve, e.g., that of theamplifier, frequency components occur at multiples of this specificfrequency. If these multiples are outside the transmission frequencyspectrum, this is known as out-of-band radiation, because signal energyis then transmitted outside the available spectrum and is thus lost forthe purposes of signal transmission because a receiver will filter outthe out-of-band radiation. In addition, the out-of-band radiation willinterfere with other transmission systems used at frequencies at whichthis out-of-band radiation occurs.

If there are new frequency components within the available transmissionfrequency spectrum, unwanted signal components are demodulated in thereceiver. This results in crosstalk. This significantly worsens thesignal quality and thus also the bit error rate of the received signal.The bit error rate indicates how many bits per received bit are detectedincorrectly. Error detecting codes are used to determine the bit errorrate. After distribution of the signals to be transmitted among thesubcarriers, the OFDM signal is like a noise signal, individualamplitude peaks being able to drive the amplifier of the transmitterinto the nonlinear range. Therefore, predistortion of the OFDM signal isnecessary so that the characteristic curve of the amplifier will nothave any effect on the spectrum of the OFDM signal.

FIG. 1 shows a block diagram of an OFDM transmitter according to thepresent invention. A data source 1 here serves to generate data. Datasource 1 here is a microphone having electronics connected to it foramplifying and digitizing speech signals converted by the microphone.Microphone 1 converts sound waves into analog electric signals, whichare amplified and digitized by electronic components connected to themicrophone. The digital data stream generated from these speech signalsleads to a source coding 2. This source coding 2 is performed by aprocessor.

Source coding 2 reduces the number of bits formed from the speechsignals by having source coding 2 remove redundancy from the digitaldata stream. By using psychoacoustic models, data not necessary forreproduction of the speech signals is removed from the speech signals.After source coding 2, the data stream thus reduced is sent to an OFDMmodulator 3. Moreover, in addition to speech signals, other data such astext data, image data and video data can also be transmitted. Thensource coding is performed specifically for the given type of data.

OFDM modulator 3 first performs a differential phase modulation of thesignals to be transmitted. Differential quadrature phase shift keying orDQPSK is used for this purpose. DQPSK is a digital modulation method inwhich the signal phase shift is modulated. The phase shift in a certaininterval of time, i.e., per bit, is used as the modulation signal. Aphase shift of ±90° is used here. Differential modulation methods havethe advantage that no absolute value need be determined in the receiverto demodulate the signals because the information transmitted iscontained in the phase shift of the signals transmitted. A bit string of110 thus leads to a phase shift of +90° each for the two ones and −90°for the zero.

In addition to DQPSK, other differential and nondifferential phasemodulation methods may also be used. However, it is also possible to useamplitude or frequency modulation methods here.

DQPSK is a complex modulation method because the bits of the bit streamcarried in OFDM modulator 3 are mapped in phase shifts. If a phase of asignal is shifted, a complex plane is used for the graphicrepresentation of the signals as vectors, a real component being on theabscissa and an imaginary component on the ordinate. A signal having aphase of >0 is rotated counterclockwise by this phase in the complexplane starting from the abscissa. If a 90° phase shift is performed fourtimes, this leads back to the starting signal. Thus, four modulationstates which can be differentiated mutually are possible with DQPSK.

In addition to differential QPSK, OFDM modulator 3 distributes thesignals to be demodulated among the subcarriers, resulting in an OFDMsignal. A complex signal is obtained as a result of DQPSK performed byOFDM modulator 3, so a first and a second data output of OFDM modulator3 are connected to a first a and second data input of a predistorter 4to separately process two components of the signal, the imaginarycomponent and the real component.

Predistorter 4 distorts the signals coming from OFDM modulator 3according to a transfer characteristic of amplifier 8. The transfercharacteristic of amplifier indicates how the amplitudes and phases ofthe amplifier output signal vary as a function of the amplitudes of theamplifier input signal. Predistorter 4 inverts this characteristic toperform the predistortion, with a linear gain factor of amplifier 8being calculated out, so that predistortion does not lead to damping ofthe signals coming from OFDM modulator 3. Predistorter 4 is implementedon a digital signal processor. The data regarding the characteristic ofamplifier 8 is obtained by predistorter 4 via a third data input from ameasurement module 12.

After predistorter 4, the predistorted signals go to an input 5. Thesignals are still complex, so that two data outputs lead frompredistorter 4 to input 5. Input 5 inserts a test signal into thepredistorted OFDM signal. Input 5 thus inserts the test signal into theOFDM signal at certain times, so that the test signal is availableinstead of the OFDM signal at these times. These times are preset, e.g.,every hour or once a day. This measurement is performed before theactual operation of the transmitter according to the present inventionand is then continued at preset times during operation of thetransmitter.

In DAB, a zero symbol is provided for synchronization at the start ofthe DAB frame with which the DAB signals are transmitted. FIG. 2 shows aDAB frame. A synchronization channel 40 at the start of the DAB framehas the zero symbol. In a fast information channel 41, informationregarding the multiplex and other service information is transmitted. Amain service channel 42 has the data to be transmitted such as audioprograms and/or multimedia data.

The test signal is input into this zero symbol so that no other datatransmitted in the DAB frame is overwritten. It is acceptable for asynchronization symbol to overwrite the zero symbol of a DAB frame witha test signal because synchronization cannot be expected to stop afterone frame because such input is relatively rare, as mentioned above. Thetest signal, which is also complex, is generated by a signal generator13. Signal generator 13 has two data outputs leading to input 5. Input 5thus receives the test signal from signal generator 13 over its thirdand fourth data inputs. Signal generator 13 is a conventional oscillatorfor generating sinusoidal oscillations. As an alternative, the testsignal may also be input upstream from the predistorter. This point isexplained in greater detail below.

The test signal must meet the following requirements. First, the testsignal must not be filtered by a module of the amplifier; therefore, avery low frequency is used for the test signal. In addition, anothercondition is that the test signal has a constant envelope. Theamplitudes of a test signal thus have the same value, so that theenvelope, which is pulled from one maximum value to the next in thepositive and negative ranges, is a parallel to the abscissa, whichrepresents the time axis. This permits a simple determination of thetransfer behavior of the amplifier by such a test signal. A sinusoidaloscillation exhibits such behavior.

The OFDM signal having the test signal input into it goes as a complexsignal over the first and second data outputs from input 5 to onedigital-analog converter 30 and 35, converting the components of thecomplex signal into analog signals which then go to a quadraturemodulator 6. The complex OFDM signal having the test signal input intoit is converted by quadrature modulator 6 into a real signal. Complexsignal y(t) is described mathematically with the following equation:y(t)=a(t)+jb(t)and is converted to a real signal x(t) by the following procedure:x(t)+a(t)cos(ωt)−b(t)sin(ωt)where ω is a frequency by which the OFDM signal is converted to anintermediate frequency by upmixing.

Quadrature modulator 6 is followed by upmixer 7, with the real OFDMsignal being converted to the intermediate frequency range. Upmixer 7therefore has an oscillator to generate the frequency by which the OFDMsignal is to be shifted.

The OFDM signal converted to the intermediate frequency is sent toamplifier 8 after upmixing 7, or it is amplified according to thetransfer characteristic of amplifier 8. After amplifier 8, the OFDMsignals go first to an antenna 9 which transmits them, and they also goto a downmixer 10 which reduces the amplified signal back to a baseband.This component of the OFDM signal is thus fed back. This component is ofcourse very small in comparison with the component that is sent, forexample, it may amount to less than one percent, because most of thesignal energy is used for transmitting the OFDM signals. After feedback,the OFDM signal is output with a directional coupler. The directionalcoupler has two lines which are positioned so as to permitelectromagnetic output of signal energy from one line to the other line.

The baseband is the frequency range in which data is generated. Afterdownmixer 10, a complex signal is generated again from the real signalin a quadrature modulator, so that quadrature modulator 11 has two dataoutputs, one analog-digital converter 31 and 32 connected to each todigitize the complex signal components. The digitized signals then go tomeasurement module 12.

Over its first and second data inputs, measurement module 12 thusreceives the OFDM signal with the input test signal amplified byamplifier 8. Over its third and fourth data inputs, measurement module12 receives the OFDM signal having the input test signal from the firstand second data outputs of input 5. The OFDM signal having the inputtest signal carried from input 5 to measurement module 12 is storedtemporarily in measurement module 12 until the same OFDM signal havingthe input test signal is sent from quadrature demodulator 11 tomeasurement module 12. This permits a comparison of the input testsignal upstream and downstream from amplifier 8. The transfercharacteristic of amplifier 8 is determined by this comparison accordingto absolute value and phase as a function of the input amplitudes. Toperform the synchronization, measurement module 12 is connected by itsfifth data input to a third data output of signal generator 13, so thatmeasurement module 12 is notified when a test signal is generated.Measurement module 12 has a data output connected to a second data inputof predistorter 4, so that predistorter 4 predistorts signals comingfrom the OFDM modulator according to the transfer characteristic ofamplifier 8 which has been sent. Measurement module 12 operates onlywhen a test signal has been input. A processor controls signal generator13 when the test signal is generated.

FIG. 3 shows a method according to the present invention fortransmitting signals over wireless channels. Data is generated in step14. This takes place by way of a microphone, as described above.However, other data sources are also possible, including a computer witha keyboard. Source coding is performed in step 15, with redundancy whichis not necessary for reconstruction of the speech data in the receiverbeing removed from the speech signals. In step 16, modulation of thedata stream after source coding 15 is performed, differential phasemodulation being performed here as described above.

In step 17, the data stream is distributed among various subcarriers byOFDM modulation. In step 18, predistortion is performed according to thetransfer characteristic of amplifier 8. In step 19, a test signal isgenerated. In step 20, the test signal is input into the predistortedOFDM signal at certain times, namely at the location of the zero symbol.In step 43, a digital-analog conversion of the OFDM signal having thetest signal is performed. In step 21, quadrature modulation is performedto obtain a real signal from the complex signal.

In step 22, the real signal is converted into the intermediatefrequency. In step 23, amplification of the converted signal isperformed by amplifier 8. In step 24, the amplified signal is sent,while a portion of the amplified signal is again mixed down in step 25and converted back to a complex signal a quadrature demodulator in step26. Analog-digital conversion of the complex signal is performed in step44, so that the same test signal that was input and the test signal sentthrough amplifier 8 can be compared in step 27 to determine the transfercharacteristic of amplifier 8. If no test signal is input, the methodends here. In step 28 the predistorter is adjusted according to thetransfer characteristic of amplifier 8 thus determined. The method endswith step 29.

The amplitude of the test signal which is input in various DAB frames isincreased incrementally to completely run through the characteristic ofamplifier 8. The entire transfer characteristic of amplifier 8 is thusdetermined.

As an alternative, a test signal having an envelope that is not constantis input into the OFDM signal. The envelope of the test signal isadjusted so that the control settings of amplifier 8 are fully adjusted.The transfer characteristic of amplifier 8 is determined by samples ofthis test signal.

In an alternative, the test signal can be input upstream frompredistorter 4, in which case predistorter 4 is loaded with constantvalues, so that predistorter 4 then has a known effect on the signalwhich can be calculated out. Ideally, predistorter 4 does not then alterthe signal.

1. A transmitter for sending a signal over a wireless channel,comprising: a modulator for modulating the signal to produce a modulatedsignal that is distributed over a subcarrier; a predistorter forpredistorting the modulated signal distributed over the subcarrieraccording to an amplifier transfer property in order to produce apredistorted signal; a mixer for converting the predistorted signal froma baseband frequency into an intermediate frequency in order to producea converted signal; an amplifier for amplifying the converted signal inorder to produce an amplified signal; an antenna for sending a firstportion of the amplified signal; a mixer for mixing a second portion ofthe amplified signal down from the intermediate frequency to thebaseband frequency in order to produce a mixed-down signal; ameasurement module for comparing the mixed-down signal with thepredistorted signal to determine the amplifier transfer property and fornotifying the predistorter of the amplifier transfer property; a signalgenerator for generating a test signal; and an input element forinputting at preset times the test signal into one of the modulatedsignal, the predistorted signal, and the converted signal, wherein: themeasurement module compares the test signal in the mixed-down signalwith the test signal in the one of the modulated signal, thepredistorted signal, and the converted signal to obtain the amplifiertransfer property, and the signal generator generates the test signalhaving an envelope that is not dependent on time.
 2. A method forsending a signal over a wireless channel, comprising the steps of:modulating the signal to produce a modulated signal that is distributedover a subcarrier; predistorting the modulated signal distributed overthe subcarrier according to an amplifier transfer property in order toproduce a predistorted signal; converting the predistorted signal from abaseband frequency into an intermediate frequency in order to produce aconverted signal; amplifying the converted signal in order to produce anamplified signal; sending a first portion of the amplified signal;mixing a second portion of the amplified signal down from theintermediate frequency to the baseband frequency in order to produce amixed-down signal; comparing the mixed-down signal with the predistortedsignal to determine the amplifier transfer property and for providing anotification of the amplifier transfer property; generating a testsignal; inputting at preset times the test signal into one of themodulated signal, the predistorted signal, and the converted signal;comparing the test signal in the mixed-down signal with the test signalin the one of the modulated signal, the predistorted signal, and theconverted signal to obtain the amplifier transfer property; andincrementally increasing an amplitude of the test signal up to a presetsize to measure a control range of an amplifier.
 3. The method accordingto claim 2, further comprising the step of: inputting the test signalinto a synchronization symbol.
 4. A method for sending a signal over awireless channel, comprising the steps of: modulating the signal toproduce a modulated signal that is distributed over a subcarrier;predistorting the modulated signal distributed over the subcarrieraccording to an amplifier transfer property in order to produce apredistorted signal; converting the predistorted signal from a basebandfrequency into an intermediate frequency in order to produce a convertedsignal; amplifying the converted signal in order to produce an amplifiedsignal; sending a first portion of the amplified signal; mixing a secondportion of the amplified signal down from the intermediate frequency tothe baseband frequency in order to produce a mixed-down signal;comparing the mixed-down signal with the predistorted signal todetermine the amplifier transfer property and for providing anotification of the amplifier transfer property; generating a testsignal; inputting at preset times the test signal into one of themodulated signal, the predistorted signal, and the converted signal; andcomparing the test signal in the mixed-down signal with the test signalin the one of the modulated signal, the predistorted signal, and theconverted signal to obtain the amplifier transfer property, wherein anamplitude of the test signal has a size such that control settings of anamplifier are at least fully adjusted by the test signal.
 5. The methodaccording to claim 4, further comprising the step of: determining theamplifier transfer property in accordance with samples of the testsignal.
 6. The method according to claim 5, further comprising the stepof: inputting the test signal into a synchronization symbol.